There is a method of forming an image using a decolorizable toner in order to recycle a sheet. The decolorizable toner develops a color during image formation. However, when heat or the like is applied to the decolorizable toner, a color material in the toner loses a color to be decolored. If the decolorizable toner and an undecolorizable toner are combined to form an image, it is possible to erase only an image portion formed with the decolorizable toner without changing an image portion formed with the undecolorizable toner. Therefore, it is expected that an image forming apparatus that uses the undecolorizable toner and the decolorizable toner is applied to a wider variety of uses.
As a full-color image forming apparatus suitable for high-speed printing, a quadruple tandem full-color image forming apparatus is known in which four photoconductive members are arranged and developing devices including undecolorizable yellow (Y), magenta (M), cyan (C), and black (Bk) toners are respectively arranged around the photoconductive members. However, when a decolorizable toner is additionally arranged in such an image forming apparatus, if a photoconductive member exclusive for the decolorizable toner is provided, the apparatus increases in size. The quadruple tandem apparatus forms an image with all of plural toners or one or two or more toners selected out of the toners. In this case, to prevent wear and the like, the photoconductive member for an unselected toner is desirably not driven to rotate. However, if the decolorizable toner is additionally arranged in addition to the undecolorizable toner, there are a mode for forming an image with the undecolorizable toner alone and a mode for forming an image with the decolorizable toner alone. Therefore, a complicated switching structure and a complicated apparatus configuration are necessary to switch such image forming modes.